In recent years, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed landscapes and portraits, into a personal computer are rapidly becoming more popular. Additionally, portable telephones that include portable cameras that incorporate compact imaging modules with high functionality are rapidly becoming more popular. Furthermore, including an imaging module in compact information terminal equipment, such as PDAs (Personal Digital Assistants), is becoming popular.
In such devices that include an imaging function, an image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used to provide the imaging function. Recently, advancements in the miniaturization of such image pickup elements have been rapidly increasing. This has resulted in a desire for the main body of such devices and the imaging lens system used in the imaging module to also be further miniaturized. Additionally, image pickup elements with a larger number of pixels in the same area have been developed in order to achieve higher image quality, which creates a demand for higher resolution lens systems that are still very compact, as well as higher contrast performance.
Japanese Laid-Open Patent Application 2000-258684 describes exemplary single focus imaging lenses for such devices that include only two lens elements.
As stated above, recent image pickup elements are smaller and provide more pixels in a given detector area, which helps meet demands of higher resolution and more compactness that are especially required in imaging lenses for digital cameras. On the other hand, considerations of small cost and compactness have been the main considerations for imaging lenses for compact information terminal equipment, such as portable telephones with cameras. However, more recently, such devices have incorporated megapixel detectors (detectors that detect one million or more pixels), indicating increasing demand for higher performance in these devices as well, which has been accompanied by demands to make such devices smaller and to improve other performance properties. Therefore, development of lens systems with a wide range of applications based on properly balancing considerations of cost, performance, and compactness is desired.
For example, as an imaging lens for compact information terminal equipment having a large number of pixels, there has been developed a lens system having three lens components, each of which may be a lens element, with at least two lens elements being made of plastic, while the third lens element may be made of plastic or glass. However, in order to meet recent demands for greater miniaturization, a lens that uses a smaller number of lens components and lens elements, but which is equivalent in performance to these conventional lenses, is desired.
Although the lenses described in Japanese Laid-Open Patent Application 2000-258684, referenced above, each have a two-component, two-element lens construction, which includes aspheric surfaces, a lens system that is even more compact and higher in performance than this is desired. Particularly, when a small-size image pickup element is used, a lens system that well corrects lateral color is desired as the lens system because lateral color readily becomes noticeable.
Additionally, single focus lenses for compact information terminal equipment are generally of a standard type, for example, having a focal length of thirty-five millimeters in terms of standard thirty-five millimeter film conversion formats, so that single focus telescopic lenses for compact information terminal equipment with more limited maximum angles of incidence of principal rays have not been well developed. However, such single focus telescopic lenses that are compact and achieve high-performance in such compact information terminal equipment are desired in order to respond to various needs of users of such equipment.